Nothing. Your antenna will pick up digital just fine, although it is directional. Meaning if you move the antenna you will pick up a station, if you move it to a different location, you may loose it, but pick up a different station.
They do make those flat square antennas (As seen on TV - commercials), that do work quite well. We have 2 in our home to watch over-the-air television. We purchased them from Best Buy. The advantage to them is, they are all directional.
They do make a similar one that can be mounted on your RV antenna by removing the old old batwing, but I think the little square portable ones will probably work just fine also.
Or do nothing, and just use your batwing style antenna. My mother had an old over-the-air antenna on her house. It had only 1 prong left on it. I plugged it into her television, and she got a bunch of channels - Digital. Worked OK until a tree branch finally fell and knocked over the entire antenna. I got her a simple rabbit ear antenna, and she's been using it ever since. It pulls in about 45 digital channels.